Operational amplifier having improved slew rate/bandwidth characteristics

Abstract

An operational amplifier includes in its first stage current splitting means which enables the slew rate to be varied independently of bandwidth. A pair of dual collector transistors are coupled between the amplifier's input devices and a current mirror circuit. Only one collector of each of the split collector devices is coupled to the current mirror circuit and the current flowing therethrough is controlled by varying the area of the second collector. Slew rate and bandwidth are then controlled by proper selection of current and the ratio of the collector areas.

We claim: 
     
       1. An amplifier having first and second stages, comprising: first and second field-effect-transistor input devices adapted to receive first and second input signals;   a first current source coupled to said first input device for supplying current thereto;   a second current source coupled to said second input device for supplying current thereto;   first and second current splitting means coupled to said first and second input devices respectively and to said first and second current sources respectively for splitting current supplied thereto by said first and second current sources respectively; and   current mirror means coupled between said first and second current splitting means and coupled to said second stage for supplying current to said second stage when said first input device is on and said second input device is off and for receiving current from said second stage when said second input device is on and said first input device is off, said first current splitting means comprising transistor means having an emitter coupled to said first current source, a first collector coupled to said current mirror means, a second collector adapted to be coupled to a first source of voltage and having a base electrode adapted to be coupled to a bias potential, said first and second collectors being of different area.   
     
     
       2. An amplifier according to claim 1 wherein said second current splitting means comprises second transistor means having an emitter coupled to said second current source, a base adapted to be coupled to a bias potential, a first collector coupled to said current mirror means and to said second stage, and a second collector adapted to be coupled to said first source of supply voltage. 
     
     
       3. An amplifier according to claim 2 wherein said first and second transistor means are dual collector transistors. 
     
     
       4. An amplifier according to claim 2 wherein said first and second input devices comprise respectively first and second junction field-effect-transistors having common source electrodes, said first junction field-effect-transistor having a gate electrode adapted to receive said first input signal and a drain electrode coupled to said first current source and to the emitter of said first transistor means, said second junction field-effect-transistor having a gate electrode adapted to receive said second input signal and a drain electrode coupled to said second current source and to the emitter of said second current means. 
     
     
       5. An operational amplifier for receiving first and second input signals and having an output, said operational amplifier having a bandwidth and a slew rate, comprising: first and second field-effect-transistor input devices adapted to receive first and second input signals;   first means for supplying current to said first and second input devices;   second means coupled to said first means and to said first and second input devices for enabling said slew rate and said bandwidth to be varied independently of each other; and   current mirror means coupled to said second means and to said output, said second means comprising first and second transistor means each having a first collector coupled to said current mirror means and a second collector adapted to be coupled to a source of voltage for splitting current supplied thereto by said first means.   
     
     
       6. An operational amplifier according to claim 5 wherein each of said first and second transistor means comprise dual collector transistors. 
     
     
       7. An operational amplifier according to claim 6 wherein said first means comprises: a first current source adapted to be coupled between a second source of supply voltage and the emitter of said first transistors means; and   a second current source adapted to be coupled between said second supply voltage and the emitter of said second transistor means.   
     
     
       8. An operational amplifier according to claim 7 wherein said first and second input devices comprise respectively first and second junction field-effect-transistors having common source electrodes, the gate of said first junction field-effect-transistor being adapted to receive said first input signal and the gate of said second junction field effect transistor being adapted to receive said second input signal, the drain of said first junction field-effect-transistor being coupled to said first current source and to the emitter of said first transistor means, and the drain of said second junction field-effect-transistor being coupled to said second current source and to the emitter of said transistor means.